Method for strengthening reservoir fractures

ABSTRACT

Hydrocarbons are produced from hydrocarbon bearing formations, including oil shale, in situ through fractures communicating with production wells by injecting combustion supporting materials such as an oxygen-containing gas substantially completely throughout the fractures and then initiating in situ combustion (supported by the injected gas) at an extremity of the fractures prescribed by either injection or production wells or a subterranean detonation zone or cavity and directing the resultant in situ combustion front along the axis of the fractures and maintaining combustion at a level sufficient to fuse the formation adjacent the fractures whereby the resistance of the fractures to collapse under compressive stress is increased. Collapse of fractures treated in this manner under the influence of formation expansion promoted by subsequent temperature elevation accompanying retorting is substantially retarded. Therefore the strengthened fractures can be employed to facilitate heat transfer throughout the formation.

United States Patent [72] Inventor Harry W. Parker Bartlesville, Okla.

[21] Appl. No. 773,989

[22] Filed Nov. 7, 1968 [45] Patented Dec. 28, 1971 [73] AssigneePhillips Petroleum Company [54] METHOD FOR STRENGTHENING RESERVOIR [56]References Cited UNITED STATES PATENTS 1,361,282 12/1920 Nolan 166/2882,771,952 11/1956 Simm 166/288 X 2,780,449 2/1957 Fisher et a1. 166/2592,796,935 6/1957 Bond 166/260 3,072,188 1/1963 Morse 166/288 e HI lllIll

Primary Examiner-Stephen J. Novosad Attorney- Young and Quigg ABSTRACT:Hydrocarbons are produced from hydrocarbon bearing formations, includingoil shale, in situ through fractures communicating with production wellsby injecting combustion supporting materials such as anoxygen-containing gas substantially completely throughout the fracturesand then initiating in situ combustion (supported by the injected gas)at an extremity of the fractures prescribed by either injection orproduction wells or a subterranean detonation zone or cavity anddirecting the resultant in situ combustion front along the axis of thefractures and maintaining combustion at a level sufficient to fuse theformation adjacent the fractures whereby the resistance of the fracturesto collapse under compressive stress is increased. Collapse of fracturestreated in this manner under the influence of formation expansionpromoted by subsequent temperature elevation accompanying retorting issubstantially retarded. Therefore the strengthened fractures can beemployed to facilitate heat transfer throughout the formation.

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METHOD FOR STRENGTHENING RESERVOIR FRACTURES BACKGROUND OF THE INVENTIONThis invention relates to a process for the in situ production of oilfrom oil shale by pyrolysis with hot gases. ln accordance with anotheraspect, this invention relates to an improved method of obtaining oilfrom oil shale around a nuclear produced chimney by burning thefractures therethrough in an oxygen-enriched atmosphere, and thereafterproducing the formation by the use of hot inert gases.

Tremendous deposits of oil shales occur in Colorado, Utah, and Wyoming,and various petroleum companies and the federal government are doingresearch on methods of producing oil from these deposits. Numerousproposals have been made, including mining the shale and retorting themined shale above ground and applying heat to the shale in situ with hotgases including oxygen and excluding oxygen. Steam, hot combustion gas,hot air, etc. have been proposed as heating media for the pyrolysisoperation.

In situ combustion and in situ retorting by either in situ combustion orinjection of heat exchange fluid has been the subject of considerableexperimental investigation as means for expediting the secondaryrecovery and have found commercial application in numerous instances. Itis recognized that during the course of these secondary recoveryprocedures the formation strata is subjected to considerable expansiondue to temperature elevation. This expansion generally results in thecollapse and termination of relatively unstable communication networks,e.g., fractures, throughout the formation with the result that fluidflow to recovery wells and from injection wells is diminished ordeterred by an extent necessitating increased driving forces, i.e.,differential pressure between input and production wells. lt is quiteunderstandable that this collapse also deters natural drainage ofhydrocarbons from retorted formation where the hydrocarbon fluids arenot subjected to artificial driving forces.

The utilization of such fluid communication channels is particularlyattractive in highly fragmented or fractured formations such as thosewhich result from the subterranean detonation of explosive charges.Although the formation rock in the immediate vicinity of the detonationzone or chimney is fragmented sufficiently to enable ready access oregress of fluids, the adjacent formation is fractured to a lesser extentand generally comprises a relatively unstable network ofintercommunicating fractures which are either substantially restrictedor completely closed by thermal expansion of the adjacent formationunder the influence of elevated temperatures necessary to retort thoseportions of the hydrocarbon bearing strata. Throughout the specificationand claims the term hydrocarbon is intended to include not onlycompounds of carbon and hydrogen, but also other formation organicmatter such as the kerogen contained in oil shale from whichhydrocarbons and other substances are formed by heat.

The use of nuclear explosives to fragment underground formations hasgained considerable acceptance as an economically feasible method ofproducing oil and gas from reservoirs having such low originalpermeability as to be incapable of economic production in the originalstate. The utilization of nuclear explosives in this regard is describedbriefly by D. B. Lombard in his article "Recovering Oil from Shale withNuclear Explosives" published in Aug. 1965, issue of Journal ofPetroleum Technology, pages 877-882.

By this method a nuclear charge is placed at the desired elevation in asuitable reservoir strata and detonated to produce a cavity containingfragmented reservoir rock, the dimensions of the cavity and the extentof fragmentation depending, of course, upon the magnitude of thedetonation and the characteristics of the surrounding formations. Forexample, Lombard refers to the effects of detonating nuclear chargeshaving energies of from to about 100 kilotons in hard rock and indicatesthat the resulting fragmentation zone i.e., nuclear chimney, has adiameter of from to several hundred feet and a vertical extent of about2% cavity diameters measured from the point of detonation to the chimneytop.

It is further pointed out by Lombard that although the fragmentationzone or nuclear chimney is fairly well defined, that the sidewalls,i.e., the remaining unfragmented formation defining the nuclear chimney,possess numerous fractures extending outwardly in all directions fromthe sidewalls for a distance of approximately one-half of the diameterof the fragmentation zone. These fractures result in a substantialincrease in the permeability of the formation surrounding thefragmentation zone which enable the egress of formation fluids from thefragmentation zone and the strata immediately surrounding the nuclearchimney during subsequent retorting operations and correlary proceduresinvolving the use of elevated temperatures and pressures within thefragmented area.

The degree of fracturing and consequently the degree of permeabilitywhich results in those strata defining the outer periphery of thefragmentation zone depends primarily on certain characteristics of theformations, per se, and to some extent on the intensity of thedetonation.

It is therefore one object of this invention to provide a method fortreating fragmentation zones produced by subterranean detonation.

It is another object of this invention to improve the production ofreservoir fluids from fragmentation zones resulting from subterraneandetonations.

It is yet another object of this invention to provide a method fortreating subterranean nuclear chimneys and fractures resultingtherefrom.

It is another object of this invention to provide a method fordecreasing the permeability of fractures extending and surroundingnuclear chimneys.

It is yet another object of this invention to improve the ultimaterecovery of reservoir fluid from strata fragmented by subterraneandetonation.

It is another object of this invention to provide a method for utilizingthe heat retained in retorted subterranean fragmentation zones.

SUMMARY OF THE INVENTION ln accordance with the invention, the fracturesor channels extending outwardly from the periphery of a fragmentationzone are contacted with an oxygen-containing gas under combustionconditions to form slag channels between the fragmentation zone andinput and output wells spaced around the periphery of the fracturesextending out from the fragmentation zone.

In accordance with one embodiment of this invention, the fracturesextending outwardly from a fragmentation zone are contacted by anoxygen-enriched gas injected through wells drilled into the fracturesextending outwardly from the fragmentation zone whereby the fracturesare burned outwardly by counterflow combustion to form slag channels.Oil is produced from the formation by passing gases through thefragmentation zone to be heated and then through the channels and up thesurrounding wells positioned near the periphery of the fractures.

ln accordance with a further embodiment of the invention, to preventbypassing of some of the fractured channels an inert gas can be injectedthrough some of the fracture channels while others are being burned byan oxygen-enriched gas.

More specifically, in the production of oil shale, a nuclear device isdetonated at a sufficient depth to produce an upwardly extending chimneycontaining Kerogen, which can be retorted under known conditions toproduce shale oil. This nuclear detonation also produces fractures inthe adjacent strata. This strata normally contain as much shale oil asthe chimney, but cannot be produced since normal retorting procedurescause the collapse of the strata adjacent the chimney. Thus, accordingto this invention, a plurality of peripheral wells are placed at thefurtherest extremities of the fractures produced by the nuclearexplosion, and then an oxygen-enriched gas is injected into thefractures from the injection wells, producing the combustion of theKerogen contained within the fractures, commencing adjacent the chimneyand proceeding back toward the injection wells, and the molten Kerogenproduces a slag that, when cooled, forms a fused lining on the interiorof the fractures. The strata in between the nuclear chimney and theperipheral injection wells can now be retorted employing normaltechniques and the slag-lined fractures form channels for the productionof the oil shale, which will not collapse under the retortingconditions.

The cost of breaking oil shale via nuclear explosives can be cut by afactor of to if the shale which is only fractured can be retorted aswell as the shale which is reduced to broken rubble in the detonationzones. The present invention provides a method for preparing flowchannels in fractured oil shale which will not heal by thermal expansionand thus allow for more efficient production of oil from the use of asingle nuclear device.

Although the method herein described affords advantages when applied toall types of hydrocarbon bearing formations in which it is desired tosubstantially preserve the integrity of fractures and/or channels withinthe formation strata, it is particularly advantageous when applied toformations which have been extensively fractured by the detonation ofexplosive charges. For this reason the method is described with relationto the recovery of oil from strata fractured by such a subterraneandetonation.

BRIEF DESCRIPTION OF THE DRAWING companying injection and productionwells.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Such a formation andaccompanying injection and production of wells are illustratedschematically in the drawing wherein hydrocarbon bearing strata l ispenetrated by wells 2, 3, and 4 is occupied by detonation zone orchimney 5 and outwardly extending fractures 7. Wells 2 and 3 are placedso as to communicate with fractures 7 and thereby indirectly communicatewith the nuclear chimney 5 and the intervening strata.

Prior to the treatment of this intervening strata the highly fragmentedreservoir rock 6 in nuclear chimney 5 can be retorted by any one ofseveral methods, some of which have already been discussed. For example,the fragmented shale in such chimneys can be retorted by the passage ofa flame front from the uppermost portion of the fragmented oil shaledownwardly through the entire mass of fragmented rock whereby liberatedhydrocarbons flow downwardly through the fragmentation zone andaccumulate in the lower extremities thereof. The temperature of thecombustion zone is controlled by recycling, for example, 3 to 5 volumesof gas for each volume of air injected. These accumulated hydrocarbonsare then preferably produced by means of a production well drilleddirectionally downwardly and laterally into the lower extremities of thefragmented area. Such production of the accumulated fluids can beaccomplished by any conventional means. These methods can be made moreefficient by operating at a positive pressure over the accumulated fluidin order to prevent the vaporization thereof by the elevated temperatureencountered during shale retorting.

Elevated operating pressures, e.g., 100 to 1,000 p.s.i.a., are desirablein some processes to retort nuclear chimneys such as when hot shale gasis recycled to retort the nuclear chimney to reduce the cost ofcompressors and wells. One embodiment of the hot shale gas recycleprocess is described in copending application Ser. No. 64l,8l5. Theprocess of this invention has several advantages when employed incombination with the retorting process described in copendingapplication Ser. No.

639,490 now US. Pat. No. 3,490,202: the shale would be preheated and theamount of air necessary to retort the chimney would be reduced.

Following this procedure whereby the fragmented rock 6 within thenuclear chimney is retorted, it is desirable to retort the shaledisposed between the outer periphery of the fragmentation zone and theadjacent wells 2 and 3 taking advantage of the fluid communicationnetwork defined by the series of fractures 7. However, if thetemperature of this adjoining formation is elevated to a pointsufficient to retort the formation rock that strata expands due to itsthermal coefficient of expansion and plastic deformation characteristicsby an amount sufficient to compress and collapse the fractures, therebydestroying the network of fractures from which fluids could otherwise beinjected into the formation and through which exuded hydrocarbons couldbe directed to the production wells 2 and 3. The method of thisinvention reduces the extent of collapse associated with the elevationof formation temperature by fusing and cooling the fracture sidewallsthereby greatly increasing their resistance to collapse under theinfluence of compressional forces.

By this procedure an oxidizing material such as air is injected into thefractures through either of the wells 2 or 3 and combustion or formationhydrocarbon is instituted in the formation immediately adjacent thefractures and propagated by the addition of the oxidizing materialthereto with the result that the temperature of the formation rockdefining the fractures; i.e., the fracture sidewalls, is elevatedsufficiently to fuse the same. Following a sufficient degree of burningto accomplish the extent of fusion desired the combustion is terminatedby discontinuing the injection of oxidizing gas and the formationsidewalls are allowed to cool to a point below their fusion temperature.It is also preferably to restrict the high temperature fusion zone tothe vicinity of the fracture sidewalls. This can be accomplished byjudiciously controlling the pressure on the combustion supporting mediumat a level sufficient only to force the gas into the fractures andpreferably not more than p.s.i. above formation pressure.

In initiating combustion in the strata adjoining the nuclear chimney andin the subsequent retorting of that strata it is desirable to takeadvantage of the heat stored in the detonation zone by virtue of theprevious retorting of that area of the formation. As a result, it ispresently preferred to inject the oxidizing material, i.e., air, intoinjection wells 2 and/or 3 and to force the gas outwardly through theseries of fractures 7 to the periphery of the chimney at which point thereservoir is at a temperature sufiicient to autoignite the hydrocarbonretained in the strata adjacent the chimney. This temperature should beat least about 600 F. and is usually in the range of from about 750 F.to about l,000 F. shortly after the termination of retorting proceduresemployed to produce the hydrocarbon in the chimney 5. Injection ofoxidizing gas is continued at a rate sufficient to maintain the in situcombustion of hydrocarbons radially outwardly from the chimney towardthe injection well until the burning has progressed to the immediatevicinity of the injection well bore or until the desired degree offusion along the fracture walls has been achieved.

Air injection rates will, of course, vary considerably depending uponthe extent of fracturing and the extent of burning maintained at anygiven time. However, exemplary of the injection rates that can beemployed in such operations are those within the range of about 50,000to about l00,000 standard cubic feet per hour of atmospheric air. Theseinjection rates are sufficient to maintain a degree of combustion alongthe fracture of the sidewalls such that the temperature of the sidewallsis elevated to a point above the fusion point of the hydrocarbon-bearingformation. For example, when the formation is oil shale the fracturesidewalls should be elevated to a temperature of at least aboutl,400-2,000 F. to accomplish this purpose. It is also possible by thismethod to treat selected portions of the fractured strata by sealing offthe injection well bore as illustrated in the drawing by means ofpackers l0 and 11. The oxidizing fluid is then injected by way of pipe10 to the space intermediate the packers and consequently enters thosefractures which communicate with the well bore in the region defined bythe packers. During such selective treatment procedures an inert gas canbe injected through pipe 9 to prevent airflow and undesired combustionin these areas. Fluids are produced from well bore 4 as desired tocontrol the pressure in the nuclear chimney.

Following this method of preparation the fractured formation can beretorted to recover hydrocarbons by any one of the numerous retortingprocedures known to the art several of which have already beendiscussed. Retorting temperatures are generally within the range of fromabout 750 to about 1,000 F. and should, of course, be maintained belowthe fusion point of the formation strata in order that the structuralintegrity of the formation and the fluid communication network definedby fractures 7 is not destroyed. Steam or nonoxidizing gas injected forthis purpose can be passed into the formation as already described byway of the fractures 7 and is preferably introduced through retortedchimney 5, and through the sidewalls of the detonation zone into theadjoining formation to take advantage of the heat stored in the chimney5. For example, in one embodiment steam or water can be injected by wayof well bore 4 into chimney 5 wherein the heat stored in the chimney istransferred to the injected water after which it is passed to thesidewalls of the chimney into fractures 7 for the purpose of elevatingthe temperature of the intermediate strata.

It is also possible to retort intermediate strata selectively in amanner analogous to that already described in relation to thepreparation of the network of fractures. For example, the heating mediumcan be injected by well bore 4 through chimney 5 into the adjoiningfracture network and passed selectively through the lower portionthereof prescribed by the blocked zone in the well bore 2 defined by theposition of packers l and 11 by blocking outlet 9. For example, theupper portions of the hydrocarbon bearing strata can be retorted in afirst stage by blocking exit 10', whereby the heat exchange mediumpasses through the upper portions of the reservoir.

After this retorting has been terminated the heat contained in thatportion of the reservoir can be employed along with the heat containedin the chimney by injecting steam or water both through well bore 4 andopening pipe and/or whereby the steam from the chimney enters the lowerportion of the formation through the nuclear chimney wall, passesthrough the formation by virtue of the intercommunicating network offractures 7 and into the area of the well bore 2.

SPECIFIC EXAMPLE A nuclear chimney is formed by a 200 kiloton bomb at adepth of 3,000 feet in an oil shale formation. The resulting nuclearchimney is 420 feet in diameter and 1,000 feet high. Fractures inducedby preparation of the nuclear chimney extend outwardly from the centerof the nuclear chimney for a distance of 500 to 600 feet.

The ring of 15 wells is drilled concentric to the nuclear chimney with aradius of 500 feet. The wells are cased and cemented. At intervals of 40feet starting from a position adjacent the top of the nuclear chimneythe casing is perforated or cut and packers set so oxygen can beinjected. Oxygen is injected at a rate of 80,000 standard cubic feet perhour. When the oxygen injected into the fractures reaches the hotpreviously retorted nuclear chimney the adjacent oil shale is ignitedand burns very intensely, melting the adjacent shale matrix andenlarging the fracture. The combustion zone burns back to the injectionwell in 5 to 8 hours, resulting in a flow channel propped with moltenshale which has solidified. The flow channels are about 2 inches wideand 5 to 10 feet across. This process of forming flow channels isrepeated at each 40-foot interval in each of the 15 wells. As a safetyprecaution, inert gas is injected into the previously formed flowchannels. After the flow channels are formed, the shale adjacent thechannels is retorted by injection of hot groduced shale gases throughthe chimney which flow throug the propped fractures to t e surroundingwells to cause heating of the shale around the propped fractures toproduce hydrocarbons. The bottom hole temperature of the surroundingwells is maintained at a temperature in the 300400 F. range byregulating the flow of gases therefrom at the surface.

lclaim:

l. A method of producing a hydrocarbon-bearing shale formation in situwhich comprises fracturing said formation, injecting a combustionsupporting material into said fractures and burning at least a portionof said hydrocarbon contained in said formation immediately adjacentsaid fractures at a rate and for a period of time sufficient to elevatethe temperature of said formation adjacent said fractures to at leastabout 1,400 F. to fuse the sidewalls of said fractures therebyincreasing the structural resistance of said fractures to compressiveforces, retorting said formation to liberate hydrocarbon therefrom andproducing the thus liberated hydrocarbon at least partially through saidfractures.

2. The method of claim 1 wherein said combustion-supporting material isan oxygen-containing gas.

3. The method of claim 2 wherein said formation further comprises asubterranean nuclear chimney and accompanying radially outwardlyextending fractures in communication with at least one injection wellbore, and said oxygen containing gas is injected into said fracturesthrough said injection well and passed through said fractures at leastto the periphery of said chimney, igniting said hydrocarbon in saidsidewalls of said fractures adjacent said chimney and continuing thepassage of said oxygen-containing gas into said fractures to direct saidcombustion radially outwardly from said chimney and axially along saidfractures to said injection well and thereby fusing the walls of saidfractures.

4. The method of claim 3 wherein the temperature of said chimney is atleast about 600 F. and said hydrocarbons are autoignited by contact withsaid oxygen-containing gas at said temperature.

5. The method of claim 3 wherein the injection pressure of said gas isup to about psi. above the formation pressure adjacent said fractures.

6. The method of claim 3 wherein said chimney is retorted prior to theinjection of said oxygen-containing gas via said injection wells and isat a temperature of from about 750 to about l,000 F.

7. The method of claim 6 further comprising retorting said formationfollowing said fusing of said fractures.

8. The method of claim 7 wherein said formation is retorted by injectinga heating medium into said formation via said chimney and saidfractures.

9. The method of claim 8 wherein said heating medium is steam producedat a temperature of from about 750 to about 1,000" F. by injecting oneof steam and water into said detonation zone to heat the same via aninjection well communicating with said chimney.

2. The method of claim 1 wherein said combustion-supporting material isan oxygen-containing gas.
 3. The method of claim 2 wherein saidformation further comprises a subterranean nuclear chimney andaccompanying radially outwardly extending fractures in communicationwith at least one injection well bore, and said oxygen containing gas isinjected into said fractures through said injection well and passedthrough said fraCtures at least to the periphery of said chimney,igniting said hydrocarbon in said sidewalls of said fractures adjacentsaid chimney and continuing the passage of said oxygen-containing gasinto said fractures to direct said combustion radially outwardly fromsaid chimney and axially along said fractures to said injection well andthereby fusing the walls of said fractures.
 4. The method of claim 3wherein the temperature of said chimney is at least about 600* F. andsaid hydrocarbons are autoignited by contact with said oxygen-containinggas at said temperature.
 5. The method of claim 3 wherein the injectionpressure of said gas is up to about 150 p.s.i. above the formationpressure adjacent said fractures.
 6. The method of claim 3 wherein saidchimney is retorted prior to the injection of said oxygen-containing gasvia said injection wells and is at a temperature of from about 750* toabout 1,000* F.
 7. The method of claim 6 further comprising retortingsaid formation following said fusing of said fractures.
 8. The method ofclaim 7 wherein said formation is retorted by injecting a heating mediuminto said formation via said chimney and said fractures.
 9. The methodof claim 8 wherein said heating medium is steam produced at atemperature of from about 750* to about 1,000* F. by injecting one ofsteam and water into said detonation zone to heat the same via aninjection well communicating with said chimney.